1. Field
This invention relates to elastomeric linings for casings of centrifugal pumps and particularly to rubber linings, especially natural rubber linings, which are constructed to prevent collapse during use.
2. State of the Art
Elastomeric linings, including rubber linings, and in particular, natural rubber linings, have been used for quite some time in centrifugal pumps, both as liners and as coverings on impellers so that the pumps could effectively handle abrasive slurries and corrosive liquids such as acids. The basic function of a liner is to provide a resistant surface to interface with fluids directed through the pump which may be abrasive or corrosive in nature. In constructing these surfaces, elastomers are often selected as fabrication materials because they exhibit these particular resistant qualities. In selecting elastomeric material, a materials problem arises in that elastomers are not sufficiently rigid in structural configurations to withstand the pressures created under pumping conditions, absent some secondary support structure. As a result, a common construction includes an outer rigid casing fabricated of some type of metal or plastic associated with an inner liner fabricated of an elastomer. Metal casings are required in situations wherein the pumping is done under high pressure conditions. The plastic casings are unsuitable for high pressure and are mainly employed for low pressure pumping conditions.
Occasionally, during use, the pump lining will collapse, that is, the rubber lining will pull away from the casing of the pump. This phenomenon is especially critical in pumps which function in high pressure environments. If this happens in a region proximate to the impeller, the impeller may chew up the lining and render the pump disabled. Even if the lining pulls away from the casing in the discharge region or in the throat region without interfering with the impeller, the sagging lining may impede flow and make the pump less effectual.
One technique for precluding the collapse of rubber linings in a large centrifugal pump is to adhere the whole outer surface of the lining to the pump casing. In U.S. Pat. No. 3,607,600 (Schreter), a method of bonding an elastomer liner to a rigid synthetic resin (plastic) casing is disclosed. The method involves casting the liner and casing as an integral unit by first laying elastomer in a mold, subsequently laying a blanket of chopped fibers over the elastomer and then applying successive layers of synthetic resin over the fibers. The synthetic layer is rolled to effect a composite layer of resin and elastomer in the interfacial region between the elastomer and resin materials. The chopped fiber extends into both the elastomer and the synthetic resin in order to provide a mechanical linkage of those two layers. In this manner, the lining is held fast to the casing so that it cannot collapse due to negative pressure within the pump.
Other attempts in this area have included the adhesion of a conventional liner to the interior of a metal casing. In this approach, the liner is coated with adhesives on its outermost surface and that surface is then abutted against the interior wall of the metal casing to form a bond.
However, neither of these techniques are generally desirable inasmuch as pump linings require periodic replacement due to the liner being worn away after extended use.
The liner/casing assembly of Schreter would require a complete replacement of the total liner and casing assembly. This replacement could prove excessively expensive. Furthermore, Schreter's assembly is not suitable for use in high pressure environments wherein metal casings are a prerequisite.
In the case of the metal casings having liners adhesively secured thereto, it is difficult to remove a lining which has been firmly adhered to the pump casing. Thus, the replacement of a lining in such an instance requires much longer time than usual and results in the pump and the system that it services being down for a long period of time.
In certain regions of the lining, for example, in the discharge nozzle, it would be possible to prevent collapse of the lining by inserting a circular, expandable band on the inside of the lining. While such a structure may prevent lining collapse, it would tend to act as an orifice, thereby restricting flow, and would tend to be eroded or corrode with time, failing before the lining failed and contributing some contamination to the liquid medium being pumped.
The problem of collapsing linings is particularly acute with soft, natural rubber pump linings which are frequently preferred in pumps handling abrasive slurries and the like. Because natural rubber is soft, it has less inherent strength than many types of pump liners. Attempts to reinforce natural rubber liners with steel mesh and the like have not been successful. Steel is difficult to prepare with no oxide layer on its surface, which tends to interfere with adhesion between the rubber and the steel. The use of brass mesh overcomes the oxide problem, but has different thermal expansion properties than rubber and thus poses another problem. Since the rubber lining normally shrinks during the curing process, the presence of brass mesh does not permit the rubber lining to shrink according to its normal manner. As a result, a reinforced rubber lining made from a particular mold no longer fits the pump casing since the lining does not shrink as it normally does.
The end result is that preexisting molds cannot be used to produce reinforced liners for the respective casings formerly supplied by nonreinforced liners produced from the same molds. The same problem is encountered with the use of hard rubber as an outer surface for the lining inasmuch as hard rubber does not shrink during curing to the same extent as soft natural rubber. Thus, from a particular mold size the lining no longer fits the casing.
If such techniques were used to reinforce a pump liner, then new molds would have to be prepared whereby the mold was of a size and a design to provide a pump lining suitable for placement within a pump casing without any shrinkage of that lining. However, even such an approach may not be successful inasmuch as it would be necessary to reinforce the whole lining in such an instance so that there is no shrinkage throughout the lining. Also, if the rubber were prevented from shrinking during curing, it would tend to be in tension in the cured lining, subjecting the rubber to faster potential wear.